Optical signal transmission has for many years being making inroads into almost every sector of data transmission. For this, the signals are typically directed via optical fibers, for example glass fibers. One of the essential difficulties to be noted is signal damping upon transmission over long distances. Amplifiers are typically incorporated into the transmission link in order to counteract signal damping.
The amplifiers most often used at present are erbium-doped fiber amplifiers (EDFAs). An EDFA is an amplifier made up of a glass-fiber segment, typically several meters long, doped with erbium. The doped glass-fiber segment behaves basically like a laser diode, and is pumped using pumps having a wavelength of 980 nm or 1480 nm.
Another amplification method has also recently been the subject of development, namely so-called Raman amplification, based on the quantum mechanical Raman effect. In Raman amplification a pump laser is typically coupled into the transmission fiber, i.e. an ordinary glass fiber, in order to bring about stimulated Raman scattering within the glass fiber.
These active optical Raman amplifiers have certain technical advantages as compared with EDFAs. These encompass at least the following:
The intrinsic noise of a Raman amplifier is much lower than that of an EDFA. The result is that a greater transmission range can be achieved for the communication systems, or a wider amplifier spacing can be implemented; this can lower acquisition and operating costs.
The amplification region can be selected largely without restriction over a wide range by adjusting the pump wavelength. In contrast thereto, the amplification behavior of an EDFA is an inherent, fixed property of erbium ions.
EDFAs become opaque to light after a failure, which can lead to a total failure of the entire transmission link. Raman amplifiers, on the other hand, lose their amplifying properties but remain transparent, so that the transmission link operates with reduced amplification but at least still remains open.
In addition, Raman amplifiers insert comparatively little polarization mode dispersion (PMD) into the transmission link, which is a great advantage over EDFAs especially for future WDM systems with a data rate of 40 Gbit/s.
Raman amplification methods are nevertheless also capable of further improvement.